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Ehud Shapiro

Publications and source records attributed to Ehud Shapiro.

At least 37 records · Page 2Linked to original sources

AI-Generated Compromises for Coalition Formation: Modeling, Simulation, and a Textual Case Study

The challenge of finding compromises between agent proposals is fundamental to AI sub-fields such as argumentation, mediation, and negotiation. Building on this tradition, Elkind et al. (2021) introduced a process for coalition formation that seeks majority-supported proposals preferable to the status quo, using a metric space where each agent has an ideal point. The crucial step in this iterative process involves identifying compromise proposals around which agent coalitions can unite. How to effectively find such compromise proposals, however, remains an open question. We address this gap by formalizing a holistic model that encompasses agent bounded rationality and uncertainty and developing AI models to generate such compromise proposals. We focus on the domain of collaboratively writing text documents -- e.g., to enable the democratic creation of a community constitution. We apply NLP (Natural Language Processing) techniques and utilize LLMs (Large Language Models) to create a semantic metric space for text and develop algorithms to suggest suitable compromise points. To evaluate the effectiveness of our algorithms, we simulate various coalition formation processes and demonstrate the potential of AI to facilitate large-scale democratic text editing, such as collaboratively drafting a constitution, an area where traditional tools are limited.

cs.MA↗

Safe Voting: Resilience to Abstention and Sybils

Voting rules may implement the will of the society when all eligible voters vote, and only them. However, they may fail to do so when sybil (fake or duplicate) votes are present and when only some honest (non sybil) voters actively participate. As, unfortunately, sometimes this is the case, our aim here is to address social choice in the presence of sybils and voter abstention. % To do so, we build upon the framework of Reality-aware Social Choice: we assume the status quo as an ever-present distinguished alternative, and study \emph{status quo Enforcing (QUE) voting rules}, which add virtual votes in support of the status quo. We characterize the tradeoff between \emph{safety} and \emph{liveness} (the ability of active honest voters to maintain/change the status quo, respectively) in several domains, and show that the voting rules are often optimal. \revision{Our characterization identifies the exact conditions under which mechanisms remain both resilient to sybils and responsive to verified participation, offering a quantitative tool for designers to measure the benefit of increased participation and verified identities.

cs.MA↗

Morpheus Consensus: Excelling on trails and autobahns

Recent research in consensus has often focussed on protocols for State-Machine-Replication (SMR) that can handle high throughputs. Such state-of-the-art protocols (generally DAG-based) induce undue overhead when the needed throughput is low, or else exhibit unnecessarily-poor latency and communication complexity during periods of low throughput. Here we present Morpheus Consensus, which naturally morphs from a quiescent low-throughput leaderless blockchain protocol to a high-throughput leader-based DAG protocol and back, excelling in latency and complexity in both settings. During high-throughout, Morpheus pars with state-of-the-art DAG-based protocols, including Autobahn. During low-throughput, Morpheus exhibits competitive complexity and lower latency than standard protocols such as PBFT and Tendermint, which in turn do not perform well during high-throughput. The key idea of Morpheus is that as long as blocks do not conflict (due to Byzantine behaviour, network delays, or high-throughput simultaneous production) it produces a forkless blockchain, promptly finalizing each block upon arrival. It assigns a leader only if one is needed to resolve conflicts, in a manner and with performance not unlike Autobahn.

cs.DC↗

Grassroots Platforms with Atomic Transactions: Social Networks, Cryptocurrencies, and Democratic Federations

Grassroots platforms aim to offer an egalitarian alternative to global platforms. Whereas global platforms can have only a single instance, grassroots platforms can have multiple instances that emerge and operate independently of each other and of any global resource except the network, and can interoperate and coalesce into ever-larger instances once interconnected. Key grassroots platforms include grassroots social networks, grassroots cryptocurrencies, and grassroots democratic federations. Previously, grassroots platforms were defined formally and proven grassroots using unary distributed transition systems, in which each transition is carried out by a single agent. However, grassroots platforms cater for a more abstract specification using transactions carried out atomically by multiple agents, something that cannot be expressed by unary transition systems. As a result, their original specifications and proofs were unnecessarily cumbersome and opaque. We enhance the notion of a distributed transition system to include atomic transactions and revisit the notion of grassroots platforms within this new foundation; present crisp specifications of key grassroots platforms using atomic transactions: befriending and defriending for grassroots social networks, coin swaps for grassroots cryptocurrencies, and communities forming, joining, and leaving a federation for grassroots democratic federations; prove a general theorem that a platform specified by atomic transactions that are so-called interactive is grassroots; show that the atomic transactions used to specify all three platforms are interactive; and conclude that the platforms thus specified are indeed grassroots. We thus provide a crisp mathematical foundation for grassroots platforms and a solid and clear starting point from which their implementation can commence.

cs.DC↗

The Blocklace: A Byzantine-repelling and Universal Conflict-free Replicated Data Type

Conflict-free Replicated Data Types (CRDTs) are designed for replica convergence without global coordination or consensus. Recent work has achieved the same in a Byzantine environment, through DAG-like structures based on cryptographic hashes of content. The blocklace is a partially-ordered generalization of the blockchain in which each block has any finite number of signed hash pointers to preceding blocks. We show that the blocklace datatype, with the sole operation of adding a single block, is a CRDT: it is both a pure operation-based CRDT, with self-tagging; and a delta-state CRDT, under a slight generalization of the delta framework. Allowing arbitrary values as payload, the blocklace can also be seen as a universal Byzantine fault-tolerant implementation for arbitrary CRDTs, under the operation-based approach. Current approaches only care about CRDT convergence, being equivocation-tolerant (they do not detect or prevent equivocations), allowing a Byzantine node to cause an arbitrary amount of harm by polluting the CRDT state with an unbounded number of equivocations. We show that the blocklace can be used not only in an equivocation-tolerant way, but also so as to detect and eventually exclude Byzantine nodes, including equivocators, even under the presence of undetectable colluders. The blocklace CRDT protocol ensures that a Byzantine node may harm only a finite prefix of the computation.

cs.DC↗

A Grassroots Architecture to Supplant Global Digital Platforms by a Global Digital Democracy

We present an architectural alternative to global digital platforms termed grassroots, designed to serve the social, economic, civic, and political needs of local digital communities, as well as their federation. Grassroots platforms may offer local communities an alternative to global digital platforms while operating solely on the smartphones of their members, forsaking any global resources other than the network itself. Such communities may form digital economies without initial capital or external credit, exercise sovereign democratic governance, and federate, ultimately resulting in the grassroots formation of a global digital democracy.

cs.NI↗

Federated Assemblies

A citizens' assembly is a group of people who are randomly selected to represent a larger population in a deliberation. While this approach has successfully strengthened democracy, it has certain limitations that suggest the need for assemblies to form and associate more organically. In response, we propose federated assemblies, where assemblies are interconnected, and each parent assembly is selected from members of its child assemblies. The main technical challenge is to develop random selection algorithms that meet new representation constraints inherent in this hierarchical structure. We design and analyze several algorithms that provide different representation guarantees under various assumptions on the structure of the underlying graph.

cs.GT↗

Grassroots Social Networking: Where People have Agency over their Personal Information and Social Graph

Offering an architecture for social networking in which people have agency over their personal information and social graph is an open challenge. Here we present a grassroots architecture for serverless, permissionless, peer-to-peer social networks termed Grassroots Social Networking that aims to address this challenge. The architecture is geared for people with networked smartphones -- roaming (address-changing) computing devices communicating over an unreliable network (e.g., using UDP). The architecture incorporates (i) a decentralized social graph, where each person controls, maintains and stores only their local neighborhood in the graph; (iii) personal feeds, with authors and followers who create and store the feeds; and (ii) a grassroots dissemination protocol, in which communication among people occurs only along the edges of their social graph. The architecture realizes these components using the blocklace data structure -- a partially-ordered conflict-free counterpart of the totally-ordered conflict-based blockchain. We provide two example Grassroots Social Networking protocols -- Twitter-like and WhatsApp-like -- and address their security (safety, liveness and privacy), spam/bot/deep-fake resistance, and implementation, demonstrating how server-based social networks could be supplanted by a grassroots architecture.

cs.DC↗

United for Change: Deliberative Coalition Formation to Change the Status Quo

We study a setting in which a community wishes to identify a strongly supported proposal from a space of alternatives, in order to change the status quo. We describe a deliberation process in which agents dynamically form coalitions around proposals that they prefer over the status quo. We formulate conditions on the space of proposals and on the ways in which coalitions are formed that guarantee deliberation to succeed, that is, to terminate by identifying a proposal with the largest possible support. Our results provide theoretical foundations for the analysis of deliberative processes such as the ones that take place in online systems for democratic deliberation support.

cs.MA↗

Grassroots Systems: Concept, Examples, Implementation and Applications

Informally, a grassroots system is a distributed system that can have multiple instances, independent of each other and of any global resources, that can interoperate once interconnected. Grassroots applications are potentially important as they may allow people to conduct their social, economic, civic, and political lives in the digital realm solely using the networked computing devices they own and operate (e.g., smartphones), free of third-party control, surveillance, manipulation, coercion, or rent seeking (e.g., by global digital platforms such as Facebook or Bitcoin). Here, we formalize the notion of grassroots systems and grassroots implementations; specify an abstract grassroots dissemination protocol; describe and prove an implementation of grassroots dissemination for the model of asynchrony; extend the implementation to mobile (address-changing) devices that communicate via an unreliable network (e.g. smartphones using UDP); and discuss how grassroots dissemination can realize applications that support digital sovereignty -- grassroots social networking and grassroots currencies. The mathematical construction employs distributed multiagent transition systems to define the notions of grassroots protocols and grassroots implementations, to specify grassroots dissemination protocols and their implementation, and to prove their correctness. The implementation uses the blocklace -- a generalization of the blockchain that functions as a fault-tolerant, conflict-free replicated data type.

cs.NI↗

Enabling the Digital Democratic Revival: A Research Program for Digital Democracy

This white paper outlines a long-term scientific vision for the development of digital-democracy technology. We contend that if digital democracy is to meet the ambition of enabling a participatory renewal in our societies, then a comprehensive multi-methods research effort is required that could, over the years, support its development in a democratically principled, empirically and computationally informed way. The paper is co-authored by an international and interdisciplinary team of researchers and arose from the Lorentz Center Workshop on ``Algorithmic Technology for Democracy'' (Leiden, October 2022).

cs.CY↗

Grassroots Flash: A Payment System for Grassroots Cryptocurrencies

The goal of grassroots cryptocurrencies is to provide a foundation with which local digital economies can emerge independently of each other and of global digital platforms and global cryptocurrencies; can form and grow without initial capital or external credit; can trade with each other; and can gradually merge into a global digital economy. Grassroots cryptocurrencies turn mutual trust into liquidity and thus could be a powerful means for 'banking the unbanked'. Grassroots cryptocurrencies have not been provided yet with a payment system, which is the goal of this paper. Here, we present Grassroots Flash, a payment system for grassroots cryptocurrencies that employs the blocklace -- a DAG-like counterpart of the blockchain data structure. We analyze its security (safety, liveness, and privacy) and efficiency, prove that it is indeed grassroots.

cs.MA↗

Cordial Miners: Fast and Efficient Consensus for Every Eventuality

Cordial Miners are a family of efficient Byzantine Atomic Broadcast protocols, with instances for asynchrony and eventual synchrony. They improve the latency of state-of-the-art DAG-based protocols by almost 2X and achieve optimal good-case complexity of O(n) by forgoing Reliable Broadcast as a building block. Rather, Cordial Miners use the blocklace -- a partially-ordered counterpart of the totally-ordered blockchain data structure -- to implement the three algorithmic components of consensus: Dissemination, equivocation-exclusion, and ordering.

cs.DC↗

Flash: An Asynchronous Payment System with Good-Case Linear Communication Complexity

While the original purpose of blockchains was to realize a payment system, it has been shown that, in fact, such systems do not require consensus and can be implemented deterministically in asynchronous networks. State-of-the-art payment systems employ Reliable Broadcast to disseminate payments and prevent double spending, which entails O(n^2) communication complexity per payment even if Byzantine behavior is scarce or non-existent. Here we present Flash, the first payment system to achieve $O(n)$ communication complexity per payment in the good case and $O(n^2)$ complexity in the worst-case, matching the lower bound. This is made possible by sidestepping Reliable Broadcast and instead using the blocklace -- a DAG-like partially-ordered generalization of the blockchain -- for the tasks of recording transaction dependencies, block dissemination, and equivocation exclusion, which in turn prevents doublespending. Flash has two variants: for high congestion when multiple blocks that contain multiple payments are issued concurrently; and for low congestion when payments are infrequent.

cs.DC↗

Multiagent Transition Systems for Composing Fault-Resilient Protocol Stacks

We present a novel mathematical framework for the specification and analysis of fault-resilient distributed protocols and their implementations, with the following components: 1. Transition systems that allow the specification and analysis of computations with safety and liveness faults and their fault resilience. 2. Notions of safe, live and complete implementations among transition systems and their composition, with which the correctness (safety and liveness) and completeness of a protocol stack as a whole follows from each protocol implementing correctly and completely the protocol above it in the stack. 3. Applying the notion of monotonicity, pertinent to histories of distributed computing systems, to ease the specification and proof of correctness of implementations among distributed computing systems. 4. Multiagent transition systems, further characterized as centralized/distributed and synchronous/asynchronous; safety and liveness fault-resilience of implementations among them and their composition. The framework is being employed in the specification of a grassroots ordering consensus protocol stack, with a grassroots dissemination protocol and its implementation of grassroots social networking and of sovereign cryptocurrencies, and an efficient Byzantine atomic broadcast protocols as initial applications.

cs.DC↗

Foundations for Grassroots Democratic Metaverse

While the physical lives of many of us are in democracies (one person, one vote - e.g., the EU and the US), our digital lives are mostly in autocracies (one person, all votes - e.g., Facebook). Cryptocurrencies promise liberation but stop short, at plutocracy (one coin, one vote). What would it take for us to live our digital lives in a digital democracy? This paper offers a vision, a theoretical framework, and an architecture for a grassroots network of autonomous, people-owned, people-operated, and people-governed digital communities, namely a grassroots democratic metaverse. It also charts a roadmap towards realizing it, and identifies unexplored territory for further research.

cs.CY↗

Fault-Tolerant Distributed-Ledger Implementation of Digital Social Contracts

A companion paper defined the notion of digital social contracts, presented a design for a social-contracts programming language, and demonstrated its potential utility via example social contracts. The envisioned setup consists of people with genuine identifiers, which are unique and singular cryptographic key pairs, that operate software agents thus identified on their mobile device. The abstract model of digital social contracts consists of a transition system specifying concurrent, non-deterministic asynchronous agents that operate on a shared ledger by performing digital speech acts, which are cryptographically-signed sequentially-indexed digital actions. Here, we address the distributed-ledger implementation of digital social contracts in the presence of faulty agents: we present a design of a fault-tolerant distributed-ledger transition system and show that it implements the abstract shared-ledger model of digital social contracts, and discuss its resilience to faulty agents. The result is a novel ledger architecture that is distributed with a blockchain-per-person (as opposed to centralized with one blockchain for all), partially-ordered (as opposed to totally-ordered), locally-replicated (as opposed to globally-replicated), asynchronous (as opposed to globally-synchronized), peer-to-peer with each agent being both an actor and a validator (as opposed to having dedicated miners, validators, and clients), environmentally-friendly (as opposed to the environmentally-harmful Proof-of-Work), self-sufficient (as opposed to the energy-hogging Proof-of-Work or capital-hogging Proof-of-Stake) and egalitarian (as opposed to the plutocratic Proof-of-Work and Proof-of-Stake).

cs.DC↗

Sybil-Resilient Coin Minting

We describe a distributed coin minting protocol that mints one coin per time unit for each member in a digital community. The protocol assumes that community members use a trust-graph to determine the genuineness of digital identities, and that doing so bounds the number of sybils (fake or duplicate identities) in the community, but does not completely eliminate them. The main goal of the protocol is to be resilient to the sybils that penetrate the community, in the sense that, in the long run, only genuine identities mint coins. The protocol accepts that sybils penetrate the community from time to time (by gaining enough trust within the trust-graph), yet assumes that every sybil is eventually exposed. Since coins minted by a sybil will most probably circulate by the time it is exposed, the protocol puts the responsibility for introducing a sybil onto its trusting neighbours and confiscates subsequent coins minted by them, until the coins minted by that sybil are accounted for. In particular, the protocol confiscates two coins for each coin minted by the sybil: one to recover what was wrongly minted and one as a fine for introducing the sybil in the first place. We argue that this approach constitutes a mechanism to deter the introduction of sybils into the community and to incentivize sybil hunting (using part of the confiscated money as a reward).

cs.SI↗